URL Rewritting -Rewrite engine is a software that modifies a web URL's appearance (URL rewriting). Rewritten URLs (sometimes known as short, fancy URLs, or search engine friendly - SEF) are used to provide shorter and more relevant-looking links to web pages. ...
The technique adds a degree of separation between the files used to generate a web page and the URL that is presented to the world.
Over the years I've read countless reasons for rewriting URLs -- basically cleaning them up at the Web server level so search engines can index them easier. There's as much confusion and controversy surrounding URL rewriting as Darwin's theory of evolution.